Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered; the description adds behavioral substance by naming the exact analysis performed (include tree + lookup-count verdict against the RFC limit) and the single-domain-per-call constraint. It stops short of mentioning cost, latency, or that live DNS queries back the result — the one piece of open-world behavior worth flagging.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three short sentences, front-loaded with the core action and scope, followed by the routing rule and then the cardinality constraint. Every sentence carries distinct information; nothing is repeated from the title or schema.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With an output schema present the description needn't explain return values, and with annotations present it needn't re-explain safety. What remains — what is analyzed, when to use it, what to use instead, and how many domains per call — is all covered.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and the single `domain` parameter is fully documented with an example in the schema itself. The description reinforces the key semantic that exactly one domain may be passed, which is the only real failure mode, but adds no format or validation detail beyond that.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource (analyze a domain's SPF record) plus the concrete outputs: include tree, RFC 10-DNS-lookup-limit verdict, and the `all` qualifier. It explicitly carves itself apart from the sibling email_security_check, so an agent can route without opening either schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives an explicit when-to-use condition ('only when the question is specifically about SPF') and names the correct alternative for the adjacent case ('overall is email secure grade, use email_security_check instead'). The 'one domain per call' line also rules out a misuse pattern.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.